This invention relates to an accelerometer, and more particularly, to an optical accelerometer based on the measurement of stress birefringence in an optically transparent, stress-birefringent material.
The acceleration of an object may be measured for several purposes. The velocity and position of the object may be calculated as the first and second integrals of acceleration as a function of time. The acceleration may also be used as a control parameter to ensure that the acceleration of the object does not exceed permissible limits.
A powered missile may be launched from a stationary source, from a moving object, or by firing it from a gun as a projectile that subsequently becomes powered. In each case, the downrange and lateral positions of the missile are determined by measuring its three-axis acceleration as a function of time and then calculating the second integral of the acceleration. The accelerations of the missile may be as high as hundreds or even over one thousand xe2x80x9cg""sxe2x80x9d, where one xe2x80x9cgxe2x80x9d is the acceleration due to gravity, 9.8 m/sec2. The acceleration-measurement apparatus must therefore be highly accurate and operable over a wide range of accelerations.
Available apparatus for the measurement of acceleration operates electromechanically or electrically. The electromechanical apparatus may not respond sufficiently quickly for applications such as the measurement of acceleration in a missile, particularly a missile initially fired as a projectile. Both the electromechanical and electrical accelerometers are susceptible to error induced by external environmental effects such as radiation.
There is therefore a need for an accelerometer that is accurate, fast acting, operable over a wide range of accelerations, and robust. The present invention fulfills this need, and further provides related advantages.
The present invention provides an accelerometer that is based on optical principles and optical measurements, and a method for its use. The accelerometer operates with a very high response rate and is accurate from zero acceleration to accelerations of over 1000 g""s. It is robust both electronically and mechanically. The accelerometer may function to measure acceleration in a single axis, with three of the accelerometers used to obtain three-axis acceleration values. Other embodiments provide three-axis measurement of acceleration in a single device.
In accordance with the invention, a method for measuring acceleration comprises the steps of providing an accelerometer apparatus comprising an optically transparent, stress-birefringent material, accelerating the accelerometer apparatus, and simultaneously determining the acceleration of the accelerometer apparatus from a measurement of stress-induced optical birefringence in the optically transparent, stress-birefringent material.
More specifically, a method for measuring acceleration comprises the steps of providing an accelerometer apparatus comprising an optically transparent, stress-birefringent material, a source of polarized light positioned to direct a polarized beam of light into the optically transparent, stress-birefringent material, and a detector system positioned to detect an output beam from the optically transparent, stress-birefringent material. The apparatus is accelerated, and the acceleration of the accelerometer apparatus is simultaneously determined from a measurement of stress-induced optical birefringence in the optically transparent, stress-birefringent material.
Two embodiments of the optical accelerometer are of particular interest. In a first embodiment, a 90-degree prism is formed of an optically transparent, stress-birefringent material. The prism has a first prism side and a second prism side adjacent to a right angle of the prism, and a prism hypotenuse side opposite to the right angle of the prism. A laser optical source produces a polarized beam directed normal to the prism hypotenuse side and toward an incident location of the first prism side. A diffraction grating is in contact with the first prism side at the incident location, and an imaging detector is positioned to receive a diffracted beam from the diffraction grating. An intensity detector is positioned to receive a reflected beam that travels in a reflected beam path from the laser optical source, reflects from the incident location of the first prism side, reflects from the second prism side, and passes through the prism hypotenuse side. A polarizer is positioned along the reflected beam path between the prism hypotenuse side and the intensity detector. A weight may be attached to at least one of the first prism side and the second prism side to improve the resolution of the accelerometer.
In a second embodiment, an accelerometer apparatus comprises an optically transparent, stress-birefringent material. The optically transparent, stress-birefringent material has a first side and a parallel second side. There is a partially reflecting layer on the first side of the optically transparent, stress-birefringent material, and a reflecting layer on the second side of the optically transparent, stress-birefringent material. The partially reflecting layer reflects a reflected portion of an incident beam and transmits a transmitted portion of the incident beam. A laser optical source has a polarized beam directed into the optically transparent, stress-birefringent material at an input location, so that the beam undergoes multiple internal reflections between the partially reflecting layer and the reflecting layer, with a transmitted portion of the beam energy passing through the partially reflecting layer at each reflection therefrom. There is a beam modification structure external to the optically transparent, stress-birefringent material, through which the transmitted portions of the beam energy pass. The beam modification structure includes a quarter wave plate and a polarizer. There are at least two intensity digitizing detectors, each digitizing detector being positioned to receive a respective one of the transmitted portions of the beam energy that pass through the beam modification structure. A weight may be attached to a side of the optically transparent, stress-birefringent material other than the first side and the second side. Most preferably, one of the digitizing detectors is positioned to receive a transmitted portion of the beam energy at each of the 2n reflections, where n is an integer ranging from n=1 to n=m and m is an integer expressing maximum resolution, to provide a direct digital readout of the acceleration.